Christos Tsiolkas's Man Booker-longlisted novel The Slap opens with a bang when a man at a suburban barbecue hits another parent's child.

But while some readers including, evidently, the Booker judges speak excitedly of the Australian author's bravery in tackling uncomfortable truths, others criticise the word-of-mouth hit as "offensive" and say it is full of "unbelievable misogyny". The Slap is turning out to be the most divisive Booker novel in years.

"Dull, boring and offensive," wrote one Amazon reviewer. Another criticised its "constant obsession with bodily functions, sex, and the f-word"; another wrote that "it had no heart, such terrible cynicism … I feel soiled after reading it".

The writer India Knight said she hated the book. "The whole novel has this ludicrous, comedy-macho sensibility – you get the feeling that if he'd been forced to read 'literary' fiction, Raoul Moat would have gulped it down in one sitting," said Knight.

"It's also unbelievably misogynistic, and I say that as someone who loves Flashman and Philip Roth ... There is no joy, no love, no hope, no beauty, just these hideous people beating each other up, either physically or emotionally."

The Slap is a bestseller in Australia, and UK sales are already rumoured to be colossal.

A publishing insider said the novel had sold 23,000 copies even before the Booker announcement, an almost unheard-of figure for new literary fiction from a relatively unknown author. The novel also won the Commonwealth Writers' Prize.

Neill Denny, editor-in-chief of the Bookseller, said that there "hasn't been a divisive book on taste grounds" in the Booker lineup for years.

The last time readers were really split over titles selected by judges was in 2003, when Martin Amis's Yellow Dog and Mark Haddon's The Curious Incident of the Dog in the Night-Time were both longlisted for the award and DBC Pierre's Vernon God Little went on to win it.

The former poet laureate Andrew Motion, who is chairing this year's Booker panel, defended The Slap, saying "quite unusually for a Booker book, the copy I read already had international bestseller written across it, which means that not everyone thinks it's a hateful misogynistic book".

He also took issue with Knight's comment that the novel was loveless, suggesting instead that "it's curdled love ... It's more complicated than being hate-filled. It's full of love that's gone wrong".

However, he admitted that he could "see why people might think it is misogynistic, in that the whole story is triggered by an act of male violence".

Christos Tsiolkas has been accused of misogyny and an unthinking, crude depiction of sex and violence. He, meanwhile, has lashed out against the quality of British and European writing. Might he be on to something?